Author: Larry A. DeWerd, Ahtesham Ullah Khan 👨🔬
Affiliation: Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin - Madison 🌍
Purpose: Total body irradiation (TBI) technique uses high energy photon beams with extended source-to-surface distances (SSDs) to irradiate the entire patient to prepare them for bone marrow or stem cell transplant. Accurate dosimetry for such fields can be challenging especially when the utilized dosimeters exhibit non-negligible energy dependence. Spectral information for TBI magna photon fields is required to correct for the energy dependence of a given dosimeter. The aim of this work was to leverage Monte Carlo (MC) simulations to compute energy spectra of magna photon fields at extended SSDs.
Methods: A validated phase space file for a Varian TrueBeam linac was used along with the TOPAS MC code to simulate 6 MV and 10 MV photon beams with field sizes of 180x180 cm2 projected at the SSD of 450 cm. A virtual water phantom was simulated with a thickness of 40 cm and lateral size of 400 cm to ensure adequate scattering conditions. Percent depth dose (PDD) and average photon energy as a function of depth was calculated along with energy spectra at 5, 10, 20, and 35 cm depths.
Results: The PDD ratio at 20 cm to 10 cm depths was calculated to be 0.726 and 0.771 for the 6 MV and 10 MV beams, respectively. The average photon energy decreased with depth up to 20 cm due to beam hardening and then consistently increase due to the build-up of scatter photons at larger depths. At the midplane of the phantom, the average photon energy was computed to be 0.57 MeV and 1.11 MeV for the 6 MV and 10 MV beams, respectively.
Conclusion: This work successfully calculated energy spectra for extended SSD TBI photon fields. The data from this work will be utilized to investigate the energy dependence correction factors for clinically-utilized dosimeters in magna photon fields.